1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to ultra-short pulse-width solid-state lasers that operate in the eye-safe region of the spectrum.
2. Description of the Related Art
Solid-state lasers employ a doped-insulator lasing medium, which may be a crystal or glass material. The input power source to the lasing medium is pumplight energy, which is optically coupled into the medium. Solid-state lasers can be configured as amplifier stages or as laser resonators. The resonator variety are distinguished by the fact that they self-oscillate and do not require a laser beam input from another device. The pumplight energy in laser amplifiers and laser resonators (collectively “lasers”) may be derived from high power light emitting diode arrays, other lasers, or other sources that are known to those skilled in the art. Pumplight energy is used to raise the energy level of dopant ions within the lasing medium. A lasing action occurs when the ionic energy returns to it base state and, in doing so, releases light energy at the laser beam wavelength.
Solid state lasers have been designed to operate at various wavelengths, with the infrared bands proving to be particularly useful. At any given wavelength, there is some level of beam energy, or fluence, that represents a threshold of damage to the human retina. The band of wavelengths from about 1.4 microns to 1.8 microns has been shown to require energy levels that are several orders of magnitude greater, as compared to other wavelengths, before the threshold of eye-damage is reached. In fact, this band has been deemed the “eye-safe” band by certain US government agencies. Thus, in operational environments where humans are present, eye-safe lasers are preferred because they are safer.
The demand for eye-safe lasers and laser resonators is increasing, as is the desire for greater output power levels in such devices. Of course, compact size, robustness, high efficiency, high beam quality, ultra-short pulse duration, and low cost are also desirable features in eye-safe lasers. Applications for such lasers include a variety of ground-based and airborne sensing applications requiring operation at large stand-off ranges, as well as LADAR, range finding and target identification functions. Power level demands for such devices are growing from the range of about 10 millijoules per pulse up to hundreds of millijoules per pulse. Pulse widths are desired to be under one nanosecond in Q-switched laser applications.
Prior art in eye-safe lasers employed in large standoff applications are typically neodymium ion doped yttrium-aluminum-garnet (as well as other crystal hosts) lasers that are shifted to the eye-safe band using an optical parametric oscillator. Referred to by those skilled in the art as an OPO-shifted Nd:YAG laser. However, in spite of these lasers' excellent efficiency, they are inherently bulky and cumbersome, as they typically require many pumplight diode bars to operate with an appreciable energy output. Furthermore, energy conversion based on the OPO shift is inherently inefficient and results in compromised beam quality. Direct eye-safe lasers based on erbium ions are also known, but utilize an ionic energy transfer between ytterbium ions and erbium ions, both diffused in a phosphate glass host. See generally T. Yanagisawa, K. Asaka, K. Hamazu, and Y. Hirano, “11-mJ, 15 Hz single frequency diode-pumped Q-switched Er, Yb:phosphate glass laser”, Optics Lett. 26(16), 1262–1264, (2001), and also see A. Levoshkin, A. Petrov, and J. E. Montagne, “High-efficiency diode-pumped Q-switched Yb:Er:glass laser”, Optics Communications, 185, 399–405 (2000). One problem with this approach is that the glass host is severely limited by its poor thermal properties such that operating these lasers at higher average powers is prohibited. Prior art attempts to reproduce the ytterbium-erbium ionic energy transfer pumping process in a crystal host, such as YAG and others, have resulted in severely limited laser performance as indicated in T. Schweizer, T. Jensen, E. Heumann, and G. Huber, “Spectroscopic properties and diode pumped 1.6 μm performance in Yb-codoped Er:Y3Al5O12 and Er:Y2SiO5”, Optics Communications, 118, 557–561 (1995). This is due to the fact that unlike phosphate glass, the energy level dynamics of erbium in a crystal host is much less favorable as compared to glass.
The implementation of a direct resonant pumping of an erbium ion doped YAG (“Er:YAG”), and other crystal hosts, has been proven to be reasonably efficient, as was reported in K. Spariosu, M. Birnbaum, and B. Viana, “Er3+:Y3Al5O12 laser dynamics: effects of upconversion”, J. Opt. Soc. Am. B, 11(5), 894–900, (1994), and in K. Spariosu and M. Birnbaum, “Intracavity 1.549 μm pumped 1634 μm Er:YAG Lasers at 300 K”, IEEE J. Quantum Electron. 30(4), 1044–1049, (1994). However, like the resonantly pumped ytterbium laser, which is an inherently efficient system, the resonantly pumped erbium laser suffers from limited inversion density governed by the Stark splitting of the lower laser level manifold, typically the ground state of the ions. Therefore, in order to achieve ultra-short pulse-width Q-switched operation, it is necessary to devise a technique for implementing additional gain boost for optimizing this action. Thus, there is a need in the art for a technique to efficiently achieve additional gain boost in an Er:Crystal laser.